1. Field of the Invention
The present invention relates generally to the field of memory devices, and more particularly to a mask set used to fabricate memory devices and a method for fabricating semiconductor devices by using the mask set.
2. Description of the Prior Art
In the past few decades, semiconductor devices have become more and more integrated and methods used to fabricate the semiconductor devices have been improved continuously. Among all of the factors relevant to the electrical properties of the semiconductor devices, the thickness and dielectric constant of the gate dielectric layer are the most influential ones. In detail, in order to keep or enhance the drift current in the semiconductor devices, high dielectric constant (hereinafter abbreviated as high-k) materials are often used to replace the conventional silicon oxide as the gate dielectric layer. The main advantage of adopting the high-k materials is that it decreases physical limit thickness, reduces leakage current, and obtains equivalent capacitor in an identical equivalent oxide thickness (EOT).
On the other hand, the conventional polysilicon gate also has faced problems such as inferior performance due to boron penetration and unavoidable depletion effect which increases equivalent thickness of the gate dielectric layer, reduces gate capacitance, and worsens a driving force of the devices. Thus metal gates, which include metal gate electrodes and work function metals, are developed to replace the conventional polysilicon gate.
However, there is always a need in the semiconductor processing art to develop semiconductor device with superior performance and reliability. For example, there is a need to effectively fabricate complementary metal-oxide-semiconductor devices (CMOS) with metal gates, n-type work function metal layers, p-type work function metal layers and high-K gate dielectric layers.